This invention relates to production of a bone-implant for use as a prosthesis in orthopedic therapy.
It has been proposed to replace deformed or defected joints in the living human body by artificial joints. Intramedullary nails have been used in case of bone fracture of diaphysis. Such prosthesis elements implanted into the human body, that is bone-implants, have practically been made of a metallic material, typically stainless steel SUS316L, Co-Cr alloy, Co-Cr-Ni alloy and Ti-6%Al-4%V alloy, the material being cast, forged or sintered into a given shape or contour.
The bone-implants made by these metallic materials have some disadvantages. A cortical bone in the human body has a modulus of bending elasticity (hereinafter simply referred to as elasticity) in the order of 16 GPa whereas stainless steel SUS316L, Co-Cr alloy and Ti-6%Al-4%V alloy have 200 GPa, 213 GPa and 124 GPa respectively, about 8 to 13 times as large as that of the cortical bone in the human body. When some stress resulting from deformation, deflection and torsion of the artificial joint is focussed on a specific region, the cortical bone in the human body at a portion near the stressed region of the artificial joint would become destroyed due to a great difference in elasticity.
It has been found from clinical trials for cementless hip prostheses, in which an artificial replacement and the living bones are directly joined together without use of any cementing material, particularly a cement based on acrylic resin, that a stem of the artificial joint is subjected repeatedly to stress of a patient's own weight and exercise and a portion of the bone tissue surrounding the stem develops. On the contrary, other portion of the artificial bone which is hardly stressed does impart no stimulus to the surrounding bone tissue, resulting in absorption of the bone tissue and migration or loosening of fixation between a stem of the artificial joint and the living bones. Accordingly, the artificial joint could not be effectively retained in the bone tissue.
Furthermore, the artificial joints made of metallic implanting material are in general heavy and forces a great deal of burden on a patient.
Japanese Patent Publication No. 1989-148254 relates to a metallic artificial joint whose stem portion is made hollow to approach its modulus of bending elasticity to that of the living bone as well as to lighten the overall weight. However, the hollow interior of the stem portion is partly opened and communicated with the outside, which may allow blood or any other humor to be stored in the hollow interior. Storage of such humor in the artificial bone would retard healing of the affected part and provide a hotbed of bacterium.
Another disadvantage of the prior art bone-implants made of metallic material lies in a risk of corrosion of the surfaces. Once corroded in the living human body, nickel is dissolved from stainless steel SUS316L and Co-Cr-Ni alloy and vanadium from Ti-6%Al-4%V alloy, resulting in inflammation. Moreover, it has been pointed out that such toxic materials may be of a carcinogenic nature.
Another type bone-implant made of synthetic resin material has also been proposed. For example, U.S. Pat. No. 4,902,297 discloses a composite implant prosthesis comprises a core of carbon fibers extending in a lengthwise direction, an intermediate layer encasing the core and made from a braided sheath of carbon fibers and an outermost layer encasing the intermediate layer to define an outer surface contour. This bone-implant is press-formed or formed by pultrusion process. However, at present, it has not yet been confirmed that such pure plastic implanting materials can surely provide tensile strength, torsion strength, resistance to bending and other mechanical properties which will be sufficient for their prosthetic application. It has not yet been clinically known how such plastic materials deteriorate while implanted into the human body for a long period of time.